In state of the art hearing aids various degrees of directionality is standard. The directionality is normally based on a time delay between the arrivals of the sound at two or more sound openings. The delay originating from the distance between microphones is matched with a delay created in the signal processor or the delay introduced by means of a mechanical delay device within the microphone for the case of dual port microphones. The delays are designed in accordance with free field considerations and the presence of the head is not taken into account when designing the algorithms for directionality.
Known systems for fixed directionality are designed according to the least sensitivity to sounds coming from non-frontal directions under the assumption that the head does not influence the sound field. Also conventional adaptive directivity is working to minimize the acoustic noise entering the hearing aid under free field conditions by means of adaptive variations in the directivity pattern of the hearing aid as proposed Elko in U.S. Pat. No. 5,473,701. Hence, when a hearing aid user is fitted with hearing aids in both ears, the conventional directivity is intended to minimize the acoustic noise in each ear.